Add ObjectPool tag for external labels. Use LoadExternalLabel for native calls...

runtime/vm/assembler.cc

Index: runtime/vm/assembler.cc
diff --git a/runtime/vm/assembler.cc b/runtime/vm/assembler.cc
index 9d17b1b49db85872fd135fe84c0aa9589050be0d..489f09040ded130cb94e424b5a49e74d547b0b8d 100644
--- a/runtime/vm/assembler.cc
+++ b/runtime/vm/assembler.cc
@@ -267,7 +267,7 @@ intptr_t ObjectPoolWrapper::AddObject(ObjectPool::Entry entry,
 intptr_t ObjectPoolWrapper::AddExternalLabel(const ExternalLabel* label,
                                              Patchability patchable) {
   return AddObject(ObjectPool::Entry(label->address(),
-                                     ObjectPool::kImmediate),
+                                     ObjectPool::kExternalLabel),
                    patchable);
 }
 
@@ -301,7 +301,7 @@ intptr_t ObjectPoolWrapper::FindImmediate(uword imm) {
 intptr_t ObjectPoolWrapper::FindExternalLabel(const ExternalLabel* label,
                                               Patchability patchable) {
   return FindObject(ObjectPool::Entry(label->address(),
-                                      ObjectPool::kImmediate),
+                                      ObjectPool::kExternalLabel),
                     patchable);
 }